Let me see if I understand you correctly. You're saying the MS-8 is a "drop-in" replacement for the OEM amp, buy your interface harness and I'm done? So your harness provides all connections, including power, to the amp? If it's that simple, I gotta ask the price of the harness.
I also have to ask since you appear to be a fan of staying with OEM speakers and just replacing the OEM amp, if you were to upgrade the OEM speakers, do you have any "drop-in" replacement speaker recommendations for the F25 X3?

The difference is that for the F25 a center speaker input is added to the harness (F25 OEM amp is 7 channel vs the E90 OEM amp 6-channel). I do not recommend using the OEM amp 12V power source to drive the MS-8, only using a direct to battery connection for 12V. The OEM amp ground could be used for the MS-8 and it is provided at the harness.

I do not have the F25 speaker measurements but I would assume that they are the standard 4" and 8" speakers, which in that case any 4" component set of less than 50mm deep would work. An 8" woofer of less than 65mm deep ("shallow" woofer) would also work (SWS-8, Jehnert XE200, Rainbow W200 X-Plain, for example). The SWS-8 is more a subwoofer than a midbass driver, the other two woofers are more of a midbass driver.

Any of these woofers will require a more powerful aftermarket amplifier than the internal MS-8 amps, though.